Conventional household toilets with a capacity in a range of 1.6 to 3.5 gallons per flush (GPF) typically employ a round or radially symmetrical trapway. Some states have mandated that toilets with a 1.28 GPF capacity are sold in an effort to conserve water. In response to the legislation, toilet manufacturers have offered high efficiency toilets with a 1.28 GPF capacity.
Many 1.28 GPF capacity toilets have elongated trapways to provide compliant toilets while still providing acceptable flushing and bowl cleaning capabilities. The elongated trapways permit powerful flushing action, despite low water volume.